After drilling the hole and test fitting the antenna, I mark the antenna with a very fine point sharpie where I want it to line up with the top of the fender hole. Then just below that spot I will put a SMALL drop of JB Weld or epoxy and when I lower the antenna into the hole the adhesive will 'build up' around the hole and, when dry, it will look like an antenna base. Rotating the wire just when the adhesive touched the hole will give the base a more even 'rounded' look. Then just, very carefully, paint it black.